Method of producing alkali cellulose



Nov. 4, v191:58 s. E. sl-:A'MAN ETAL 2,859,210

METHOD oF PRODUCING ALKALI AcELLuLosE Filed Dec. 1o,V 195e l Qmwk ud 2,859,210 PatentedNov. 4, 1958 United States Patent Ofi-ice 2,859,210 METHOD or PRoDUcrNG ALKALI CELLULosE Stewart E. Seaman, Osprey, Fla., and Stewart L. Seaman, White Plains, N, Y.

Application December 10, 1956, Serial No. 627,388

4 Claims. n (Cl. 260-233) This invention relates toa method of producing alkacellulose to be used in the viscose process.

Pulp for making alkali cellulose .is customarily obtained by the rayon or other viscose plants from pulp mills in the form or quality known as dissolving pulp. Such pulp has a high alpha cellulose content which is obtained in the pulp mill, in the usual sulphite process, for example, by drastic cooking and `vby caustic alkali treatments at elevated temperatures but at the expense of considerable loss ofyield. The pulp is washed with water after each alkali treatment without any provision for recovery of the alkali. Following the cooking in the digesterthe washed unbleached pulp is customarily chlorinated for the purpose of substantially removing the lignin that has not been removed in thecooking operation, after which it is washed and in addition to the caustic treatments it is subjected to" onel or more sodium hypochlorite treatments for bleaching, followed in each instance by washing and finallyA it is customarily given a sulphur dioxide treatment and'is washed, after which it is sheeted and dried, cut to size and baled. The loss in yield in such process for the production of disy solving pulp may be as high as 30%, and another economic factor of considerable importance is the loss of alkali used in the treatments. Y

The alkali content of alkali cellulose for proper xanthation must be approximately and it is the practice in the rayon plant to steep the dissolving pulp in sheets of precise dimensions as obtainedfrom the pulp mill in a caustic solution having a concentrationfof 'approximately y18%, thereby converting the pulp into alkali cellulose.

For the production of paper pulp in which the presence of a certain amount of hemi-cellulose is desirable, a less drastic cooking is employed followed byV chlorination, a, relatively mild causticV treatment and bleaching, the pulp being washed after each step. Y, 1

It is the object of the present invention to produce from a low cost pulp a uniform, high alpha alkali cellulose at lless cost than is entailed in present processes, with a high yield, high activity in subsequent Vviscose treatment and a high degree of polymerization (D. P.), and which produces a viscose containing, before filtration, very few imperfectly dissolved particles, usually termed gels, ber gels, etc. and hence gives but little trouble in filuation and in the spinning operation.

The present invention contemplates that the caustic treatment of the pulp to solubilize the hemi-celluloses, that is, the beta and gamma cellulose content, pentosan type of products, etc.,A be performed under conditions which will effect the maximum production of alpha cellulose without subsequent reduction of the soda concentration and "consequent precipitation of the hemicelluloses.

In accordance with the invention the treatment to add the soda to convert the cellulose into alkali cellulose also serves to wash the high alpha cellulose and remove the dissolved hemi-celluloses and thus precipitation of the hemi-celluloses is avoided. There is no loss of alkali and the loss in yield is minimized.. .u

In brief, the invention contemplates the use of any sulphite or prehydrolyzed kraft chlorinated pulp substantially free of lignin and treating it with cold caustic,

that is, caustic soda at substantially room temperature, the concentration of whichis such as to produce maximum swelling of the bers under which condition they are subject to maximum reaction with caustic soda to dissolve the hemi-celluloses. The refined cellulose yis then pressed to the lowest practical press ratio and is washed with a stronger caustic soda which both removes additional dissolved bensi-celluloses. remaining in the pulp after pressing and adds the additional soda required for optimum press xanthation. Finally it is pressed again to the optimum press ratio for satisfactory shredding for xanthation. In practice for both caustic treatments the pulp is converted into aslurry of fibers in the caustic, and delivered into a tank where it is held at a temperature between 20 C. and 50 C. in a proper v state of agitation for the required time.

It is contemplated for this purpose to use pulpV that has been produced by either the sulphite or prehydrolyzed kraft process and which has been chlorinated for the purpose of substantially'removing the lignin which has not been removed in the cooking operation to yield a cellulose of high activity and high D. P. In other words the pulping process to this stage may be in accordance with the treatment for the production of paper pulp.

If the viscose produced from the cellulose is to be used for tire cord, for example, no bleaching is necessary and the chlorination may be followed by a mild alkali treatment under somewhat elevated temperature to remove the lignochlorine and other dark color ingredients which were dissolved by the chlorination and are not completely soluble in acid or water condition. Where bleaching for whiteness is desirable it may follow the chlorine treatment.

The pulp is then pressed and sheeted without any requirement as to sheet size or moisture content, except that the moisture content must be exactly known, and is delivered to the viscose plant where it receives the rening and alkalizing treatments above mentioned.

The unbleached pulp should preferably be cooked to a low chlorine number, experience having shown that` 21/z% to 3% chlorine number yields an unbleached pulp sufficiently purified, with optimum activityV and a suciently high degree of polymerization. Y

In this way a much lower cost pulp is supplied by the pulp industry for viscose use than has heretofore been the case. It has been customary in producing dissolving pulp for viscose use to spend considerable effort toward making a very uniform sheet of uniform thickness, dried-to a carefully controlled low and uniform moisture content and carefully cut to uniform sheet size. Low moisture content, less than 10% tends to reduce the reactivity of cellulose.

In accordance with the present invention the chlorinated pulp, either unbleached or bleached to. any desired degree, need not be put into any precise condition for shipping, with any particular attention to sheet formation,

sheet thickness, specific moisture content or specific sheet size. In these respects the usual practice for supplying paper pulp may be followed. As a matter of convenience swelling of cellulose occurs at about 91% caustic soda. At this maximum point of swelling the individual fibers i are subject to optimum reaction with caustic soda for removal of heini-celluloses. Since ordinarily, as above mentioned, the pulp prepared at the pulp mill will contain on an .average approximately 15 %.of.moisture, it has been found practical to use a concentration of caustic soda for the first treatment of around thus providing an elective concentration of substantially 9.8% which for all practical purposes produces the maximum swelling.

yIngourpreferred process we add pulp having a known amountV of cellulose and containing about of water .to a 10% caustic lsoda solution which provides an effective concentration of about 9.8% at a properly calculated consistency. yA 6% slurry is formed ina Hydrapulper or similardevice which disperseseach individual ber into fthe caustic whereby optimum treatment is obtained..

:The slurry is `dropped into anagitating tank along with enough 10% caustic to bringthe slurryto a consistency of aroundv 1.5%

The'1.5% slurry is. formedwinto a .web and passed through a suitable press, for example, that described. in copending .application ASerial No. 566,844 by which it is reduced to a very. low press-ratio of between 2 and 31/2 to'. 1, preferably around 2or'2.5 to l, in order to express the maximum amount of hemi-celluloses.

The pressed alkaline pulp' is then delivered to a second agitating tank along with asuflcient volume of approximately 16% caustic, specifically 15.3%, to provide a slurry ofabout 1.5 consistency.

The. slurry also is formed intoa web and passed through ya suitable press, .for example, .that described in said copending application, .by which it is reduced .to that press ratio suitable for shredding prior to xanthation, for example, from 21/2 :1 to 31/2 :1.

The second caustic'treatment. using. about 16% caustic removes more `hemi-cellulose from the highly swollen fibers, ,thus yielding arproductwith a lhigher alpha cellulosecontent andproviding an alkali cellulose containing around. 15% NaOH which isfbest for xanthating.

EXAMPLE I Effect on alpha cellulose content by steepngwth 11% concentration NaOH at 20 C. and/10 C.

No gels in viscoseuntreated fibers negligible.

EXAMPLE II Ejecton alpha cellulose content by sleeping with .11% 'concentration NaOH. at 20 C. with anothermakeof pulp-with three derent tests for vcheck results 11% 11% 11% NaovH, NaOH, NaOH, .20 C. 26 C. 20 C.

Alpha Cellulose: Percent Percent Percent Unbleached 86.6 86.6 86.6 g AfteLChlOrinat; ,85. 55 85.55 85.55 gAfler A1kali .r, r ..A 98,02 99. 12 98. 88

fromwhich `it passes on toa conveyor 4 EXAMPLE Effect on alpha cellulose content by steeping in severall stepsrwz'th l8%`concentraton NaOH at 20 C.

- EXAMPLE l IV Effect on alpha cellulose content by steep'ng with 10% concentration NaOH .at 20 C. followed by a second sleeping with I 6 concentration NaOH at 20 C.

Alpha cellulose, percent Unbleached 86.6 After chlorination v 85.5 After alkali 10% 98.7 After.alkali.16% 99.2

HExample IV shows the percentage of .alpha cellulose obi tained--by a treatment with a high swelling concentration of caustic .followed byy a treatment with a concentration which will provide an alkali cellulose having the required percentage of NaOH;v

y:The best dissolving pulp produced by pulp mills using conventional methods' has an alpha cellulose content of` 96% .or thereabouts and Vhas.sutfered a loss offron1-20 to 33% in passing from chlorinated pulp tothe nished product. As compared .withthis the cellulose at the end of the9'.8% slurry steeping is refined to an alpha cellulose content of 98% or more vand at the end of the 16% caustic treatment to about 99% alpha cellulose with a loss not exceeding/19% from ythe original chlorinatedV pulp.

The accompanying drawing illustrates' diagrammatically an apparatus for the continuous production of alkalicellulose in accordancewith the .method of the invention.

A weighed amount of chlorinated pulp substantially free Y of lignin isV introduced into Va pulper 1 ofa conventionaltype, such as a Hydrapulper, into which has previouslyI been introduced from a supply tank 2 a determined` amount of-10% NaOH. Aslurry of the proper consistency results `from the vigorous .agitation in pulper 1, whereupon the slurry is reduced inconsistency by additional NaOH fromY supplytank 2 tofanoptimum consistency for forming a uniform web for pressing and is discharged into steeping tank 3 in which the treatment is .continued Withetlective stirring. v

' Fromthe tank 3 the slurry is ,pumped to a .web former 4 Vand is delivered in the. form .off a continuous web into the nipof press rolls 5 ,and pressed yto a `low press ratio.

The liquor expressed from the web by pressrolls 5 is deliveredby line y,6 .to idialyzers 7.

From the rst pressS the web is delivered by conveyor 8 into a second steeping tank 9 having asuitable agitator and ,into which NaOH ata concentration of 16% .is fed by line 10 as needed to maintain the properconsistency. The treatment in ;tank.9ris.continued for substantiallythe same time as .in tank. From tank 9 thealkali cellulose slurry .is pumped to web Vformer 11 and jis 'deliveredin the form `of .a continuous. web into the nip of press rolls 12 in which -it is again .pressed to a .low press ratio and 13 for delivery to the shredder. l

The Apress .liquor from presslZ is delivered by line 14 to supply tank 2. In order'toobtain a concentration of 10% in the supply tank, sucient recovered 'caustic at `a concentration of .about 5% Ais deliveredv from the-dialyzers by line 15 into the supply tank 2.

In this way a continuous counter-current-cyclic -process is performed. From the rst press 5 the .expressed caustic soda containing the maximumamount of heini-cellulose is pumped to dialyzers where the concentration is reduced to about 5%, the hemi-cellulose is removed and the dilute 5% caustic soda is pumped to supply tank 2.

Dependent upon the alpha cellulose content of the chlorinated pulp entering the process, the percentage of hemi-cellulose dissolved during treatment will reachV a results in an alkali cellulose containing a very minimum of hemi-cellulose after pressing to contaminate the regenerated cellulose finished products. Minimum hemi-cellulose content is required to yield maximum strength and fatigue value of the yarn.

What is claimed is:

1. The method of producing alkali cellulose having a high degree of polymerization for use in the viscose process comprising first forming a slurry of a chlorinated pulp of high degree of polymerization and substantially free of lignn with a sodium hydroxide solution having an eective alkali concentration of substantially 9% and maintaining the slurry for Ysubstantially minutes at a temperature between and 50 C. and producing a maximum swelling of the ber, pressing the pulp so treated to a press ratio of between 2 and 3.5 to 1, then forming a slurry of the pressed pulp with a second sodium hy-V droxide solution of higher concentration and maintaining the slurry for substantially 15 minutes at a temperature between 20 and 50 C. and pressing the product of the second causticizing to a press ratio of between 2 and 3.5 to 1, the concentration of said second caustic solution being suicientrto produce an alkali cellulose having an alkali content of not substantially less than 15% after pressing.

2. The method dened in claim 1 in which the concentration of the caustic solution of the rst treatment is not substantially less than 9%% nor more than 11% and the treatment is continued for a period of substantially 15 minutes at not less than 20 C. nor more than 50 C. v

3. The method delned in claim 2 in which the concentration of the caustic solution of the second treatment is substantially 16% and the second treatment is continued for substantially 15 minutes at not less than 20 C. nor more than 50 C.

4. The method defined in claim 3 in which ythe caustic Y solution in both slurry treatments is free of extraneous oxidizing agent, thereby retaining a high degree of polymerization.

References Cited in the le of this patent UNITED STATES PATENTS 2,392,269 Schmitz Ian. 1, 1946 

1. THE METHOD OF PRODUCING ALKALI CELLULOSE HAVING A HIGH DEGREE OF POLYMERIZATION FOR USE IN THE VISCOSE PROCESS COMPRISING FIRST FORMING A SLURRY OF A CHLORINATED PULP OF HIGH DEGREE OF POLYMERIZATION ADN SUBSTANTIALLY FREE OF LIGNIN WITH A SODIUM HYDROXIDE SOLUTION HAVING AN EFFECTIVE ALKALI CONCENTRAION OF SUBSTANTIALLY 91/2% AND MAINTAINING THE SLURRY OF SUBSTANTIALLY 15 MINUTES AT A TEMPERATURE BETWEEN 20* AND 50* C. AND PRODUCING A MAXIMUM SWELLING OF THE FIBER, PRESSING THE PULP SO TREATED TO A PRESS RATIO OF BETWEEN 2 AND 3.5 TO 1, THEN FORMING SLURRY OF THE PRESSED PULP WITH A SECOND SODIUM HY DROXIDE SOLUTION OF HIGHER CONCENTRATION AND MAINTAINING THE SLURRY FOR SUBSTANTIALLY 15 MINUTES AT A TEMPERATURE BETWEEN 20 AND 50*. C AND PRESSING THE PRODUCT OF THE 